2415. Inconsistency between unique_ptr and shared_ptr

Section: 20.3.2.2.2 [util.smartptr.shared.const] Status: C++17 Submitter: Jonathan Wakely Opened: 2014-07-03 Last modified: 2017-07-30

Priority: 2

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Discussion:

unique_ptr guarantees that it will not invoke its deleter if it stores a null pointer, which is useful for deleters that must not be called with a null pointer e.g. 

unique_ptr<FILE, int(*)(FILE*)> fptr(file, &::fclose);

However, shared_ptr does invoke the deleter if it owns a null pointer, which is a silent change in behaviour when transferring ownership from unique_ptr to shared_ptr. That means the following leads to undefined behaviour: 

std:shared_ptr<FILE> fp = std::move(fptr);

Peter Dimov's suggested fix is to construct an empty shared_ptr from a unique_ptr that contains a null pointer.

[2015-01-18 Library reflector vote]

The issue has been identified as Tentatively Ready based on eight votes in favour.

Proposed resolution:

This wording is relative to N4296.

  1. Change 20.3.2.2.2 [util.smartptr.shared.const] p29 as indicated:

    template <class Y, class D> shared_ptr(unique_ptr<Y, D>&& r);

    […]

    -29- Effects: If r.get() == nullptr, equivalent to shared_ptr(). Otherwise, if D is not a reference type, equivalent to shared_ptr(r.release(), r.get_deleter()). Otherwise, equivalent to shared_ptr(r.release(), ref(r.get_deleter()))Equivalent to shared_ptr(r.release(), r.get_deleter()) when D is not a reference type, otherwise shared_ptr(r.release(), ref(r.get_deleter())).